1. Field of the Invention
The present invention relates to a high-frequency signal line and an electronic device provided with the high-frequency signal line, and more particularly to a high-frequency signal line preferably for use for transmission of a high-frequency signal, and an electronic device provided with the high-frequency signal line.
2. Description of the Related Art
As an example of conventional inventions relating to high-frequency signal lines, a high-frequency signal line disclosed in WO2012/073591 is known. The high-frequency signal line includes a dielectric base, a signal line and two ground conductors. The dielectric base is formed by stacking a plurality of dielectric sheets. The signal line is provided in the dielectric base. The two ground conductors are provided on the dielectric base so as to sandwich the signal line from a layer stacking direction. Accordingly, the signal line and the two ground conductors form a stripline structure.
One of the ground conductors includes a plurality of openings arranged to overlap the signal line when viewed from the layer stacking direction. Accordingly, it is less likely that capacitance is generated between the signal line and one of the ground conductors. Hence, it is possible to reduce the distance in the layer stacking direction between the signal line and one of the ground conductors, thus resulting in a reduction in the thickness of the high-frequency signal line. Such a high-frequency signal line is used for connection between two circuit boards, for example.
In order to facilitate the operation of connecting two circuit boards on the high-frequency signal line disclosed in WO2012/073591, the center portion with respect to the lengthwise direction of the high-frequency signal line could be formed into a meandering shape. FIG. 20 illustrates a high-frequency signal line 500 of which center portion with respect to the lengthwise direction has a meandering shape. In FIG. 20, a signal line 504 and ground conductors 506 and 508 in and around a line portion 502b are indicated. FIG. 21 illustrates the high-frequency signal line 500 in a state in which both ends thereof are pulled.
As seen in FIG. 20, the center portion with respect to the right-left direction of the high-frequency signal line 500 has a meandering shape. Specifically, the high-frequency signal line 500 is formed of line portions 502a-502e connected together. The line portion 502a extends in the right-left direction. The line portion 502b extends downward from the right end of the line portion 502a. The line portion 502c extends leftward from the lower end of the line portion 502b. The line portion 502d extends downward from the left end of the line portion 502c. The line portion 502e extends rightward from the lower end of the line portion 502d. 
The high-frequency signal line 500 is pulled when it is used to connect two circuit boards to each other. Specifically, the left end of the line portion 502a is pulled leftward, and the right end of the line portion 502e is pulled rightward. Then, as seen in FIG. 21, the high-frequency signal line 500 is transformed to a shape of a Z. Thus, the distance between connectors provided at both ends of the high-frequency signal line 500 is lengthened, which permits the two connectors to be connected to the respective two circuit boards easily.
Incidentally, it is difficult to satisfy both maintenance of the flexibility of the high-frequency signal line 500 and reduction of changes in the characteristic impedance of the high-frequency signal line 500. More specifically, in the high-frequency signal line 500, the signal line 504 and the two ground conductors 506 and 508 are embedded and form a stripline structure. The two ground conductors 506 and 508 are connected to each other by via-hole conductors which are not shown. In FIG. 20, the ground conductors 506 and 508 have the same shape and overlap each other in the layer stacking direction.
When the left end of the line portion 502a and the right end of the line portion 502e are pulled leftward and rightward respectively, the line portions 502b and 502d are twisted. Since the via-hole conductors are made of metal, the via-hole conductors are relatively rigid. Therefore, if a large number of via-hole conductors are provided in the line portions 502b and 502d, the deformation of the line portions 502b and 502d will be inhibited. Thus, the flexibility of the high-frequency signal line 500 will be lowered.
Then, it is considered that no via-hole conductors could be provided in the line portions 502b and 502d of the high-frequency signal line 500. In this case, however, the two ground conductors 506 and 508 cannot be connected to each other by via-hole conductors in the line portions 502b and 502d. Therefore, the potential of the ground conductors 506 and 508 in the line portions 502b and 502d will somewhat shift from the ground potential.
The center portion of the high-frequency signal line 500 has a meandering shape. Therefore, in the structure illustrated in FIG. 20, for example, the lower side of the line portion 502a and the left side of the line portion 502b are close to each other, and the portions of the ground conductors 506 and 508 near the lower side of the line portion 502a and near the left side of the line portion 502b are close to each other. As mentioned above, the potential of the portions of the ground conductors 506 and 508 near the left side of the line portion 502b somewhat shifts from the ground potential. Therefore, a potential difference is caused between the portions of the ground conductors 506 and 508 near the lower side of the line portion 502a and the portions of the ground conductors 506 and 508 near the left side of the line portion 502b, and as indicated in FIG. 20, floating capacitance is generated. Such floating capacitance causes the characteristic impedance of the high-frequency signal line 500 to shift from a predetermined value.